In well-known thread tensioning devices in the ordinary sewing machines for imparting tension to the upper thread, the tension imparted to the thread, when the thickness or size of a thread is changed by replacement of threads, is rarely altered automatically. So it is required, for making a beautiful stitching wherein a concatenation is formed almost midway in the direction of the thickness of a work fabric to be sewed, by a well balancing or harmony of tension of an upper thread and a lower thread, to manually operate the thread tensioning device at each replacing of the upper thread for suitably adjusting the tension given to the upper thread. This adjusting operation of the device needs in addition a trial sewing for confirming whether the degree of tension is appropriate, being a time-consuming work even for a skilful operator. For the unskilled operators this adjustment operation is a far troublesome work beyond imagination.
As a device of prior art for eliminating the above-mentioned disadvantages U.S. Pat. No. 4,111,140 applied by the same applicant as this application and another person, as a co-applicant, can be cited. In the device indicated hereabove, a suitable tension can be automatically imparted to a running thread which passes through the device, for forming a good looking stitches in an ordinary sewing operation, in accordance with the thickness of the thread. This prior art is very effective, so far as the simplification problem of an automatical adjustment of the thread tension is concerned. This inventor came across a new problem during his strenuous experiments for realizing the above identified invention into the production line, that is a problem which is concerned to an adjustment of tension in accordance with the variation of the thread kind. In case of changing the kind of threads in the known upper thread tensioning devices of the ordinary sewing machines, it is a well-known fact that the tension given to the thread must be adjusted by all means even when the thickness of the thread is identical. With the device disclosed in the above U.S. Pat. No. 4,111,140 the situation is the same, i.e., an adjustment is needed at the replacement of threads of different kinds, while no adjustment is required at the replacement of threads of different thickness. True simplification or provision of a handy device for thread tensioning can be realized only by solving this problem, that is providing a tensioning device capable of automatical tension adjustment not only for the thread charge between different thickness but also for the thread change between different kind.